This invention relates to an absorbent sheet suitable for use in sanitary napkins, hygienic pads, disposable diapers, medical pads, nursing breast pads, drip sheets, kitchen paper towel, household cleaning sheets, undersheets for pet animals, and the like; a process for producing the same; and absorbent articles using the same.
Various methods for fixing a superabsorbent polymer in an absorbent structure to obtain an absorbent sheet are known. For example, U.S. Pat. No. 3,070,095 discloses a process comprising, as shown in FIG. 24, spreading a superabsorbent polymer 116 over a tissue 110, superposing another tissue 111 thereon, and pressing the superabsorbent polymer into tissues by means of a roller. According to this process, however, the superabsorbent polymer is merely fixed in a layer between a pair of tissue layers, so that the process cannot be applied to fixing of a large quantity of a superabsorbent polymer. If such an absorbent sheet as shown in FIG. 24 is used as an absorbent member of an absorbent article, the superabsorbent polymer 116 would be separated from the tissues 110 and 111 with the movement of a wearer to make gaps between the tissues 110 and 111, where a liquid to be absorbed might collect.
U.S. Pat. No. 3,670,731 discloses a process comprising interposing a superabsorbent polymer between a pair of paper-like layers, followed by embossing or quilting, to thereby fix the superabsorbent polymer at prescribed sites. This process cannot be escaped from the same problems as with the above-mentioned pressing by means of a roller.
Japanese Patent Publication 59-26467 and Japanese Patent Application Laid-Opens 54-123293 and 54-141099 disclose processes in which a superabsorbent polymer is spread over a tissue having previously been wetted by spraying steam or water, so that the polymer acquires stickiness and is thereby fixed between a pair of tissues. This process achieves fixation of a super-absorbent polymer to some extent but yet cannot completely prevent fall-off of the polymer. Besides, the amount of the polymer that can be fixed is still insufficient. In addition, the superabsorbent polymer swells in layers upon liquid absorption, sometimes resulting in absorption hindrance due to gel blocking.
Japanese Patent Laid-Open 61-132697 describes a process for producing absorbent paper containing a superabsorbent polymer, in which process a superabsorbent polymer is spread over paper before being dried in the course of paper making, followed by drying. According to this process, the amount of a superabsorbent polymer which can be fixed on paper is somewhat increased but to about 10 g/m.sup.2 at the most, which is by no means deemed sufficient. In addition, the superabsorbent polymer, which is exposed on the surface of the final product, easily falls off through dynamic actions such as friction.
A method for fixing a superabsorbent polymer on a tissue, etc. via a hot-melt adhesive applied to the entire area of the tissue is also known. Although this method guarantees fixation of a superabsorbent polymer, the most part of the surface of the superabsorbent polymer is overlaid with a hot-melt adhesive and therefore hindered from absorbing and swelling.
Alternatively, it has been suggested to apply a hot-melt adhesive spirally. This method achieves efficient fixing of a superabsorbent polymer while minimizing hindrance to absorption and swelling. However, involvement of spiral application of a hot-melt adhesive makes the process and the equipment complicated. Further, because a large amount of a superabsorbent polymer is fixed in a layer, the polymer causes gel blocking on liquid absorption and is interfered with swelling.
On the other hand, absorbent sheets made of wood pulp prepared in a dry process are also known. In order to increase the strength of this type of absorbent sheets, incorporation of a chemical binder, synthetic pulp, low-melting synthetic fiber, etc. has been attempted, only to make the sheet hydrophobic and reduce the rate of absorption. If the sheet strength is low, the superabsorbent polymer swollen with a liquid will unfavorably break out of the sheet. The absorbent sheet may be overlaid with crepe paper to increase its surface strength, but this unfavorably incurs the cost. With any of these manipulations, however, fixation of the superabsorbent polymer to the wood pulp sheet is insufficient, and the problem that a superabsorbent polymer readily comes off still remains. There is another problem that the absorbent sheet cannot be strongly compressed without reducing its rate of liquid absorption.
A process for producing an absorbent sheet comprising in-situ polymerization to obtain a superabsorbent polymer as fixed on nonwoven fabric support is also known. However, where nonwoven fabric made of hydrophilic fibers is used, a particulate superabsorbent polymer cannot be produced, and the resulting polymer is fixed over the entire surface of the nonwoven fabric substantially uniformly and has a reduced liquid absorptivity. Where nonwoven fabric made of hydrophobic fibers is used, although a particulate superabsorbent polymer can be produced, the absorbent sheet unfavorably has a low rate of absorption unlike the nonwoven fabric comprising hydrophilic fibers because of its hydrophobic properties as a whole. Besides, in-situ polymerization is unavoidably accompanied by remaining of unreacted monomers, which limits the utility of the resulting absorbent sheet for the safety consideration.
U.S. Pat. Nos. 4,605,402 and 5,021,050 disclose absorbent members which are prepared by overlaying a fiber layer on a fiber web having a superabsorbent polymer spread thereon and have a structure in which the polymer is distributed and adhered to the fiber in the middle portion in the thickness direction of the absorbent member. Although prepared to be thinned under compression, these absorbent members are not formed into a sheet and therefore too thick to be used for various purposes. Further, due to a low density, the surface of these absorbent members exhibits low absorption performance. In addition, when absorbing liquid and getting wet, the absorbent members recover its original thickness by means of the resilient force of synthetic fibers so as to obtain absorbing spaces. Thus, these absorbent members are insufficient for obtaining a thinner absorbent article.